The present embodiments relate to a method for operating a coil, through which a varying current flows.
An electric current always produces a magnetic field. In an electric coil, this magnetic field is especially marked. If another external magnetic field is present, an interaction results, and an attraction or repulsion of the coil results. This physical effect is well known and is consciously exploited in many applications. Coils are also used in various forms and with various functions in a magnetic resonance tomography system.
Magnetic resonance tomography (e.g., nuclear spin tomography) is a technique that has become widespread for the acquisition of images of the interiors of the bodies of living objects under investigation. In order to obtain an image using this method (e.g., a magnetic resonance record of an object under investigation), the body of the patient or the part of the body under investigation is exposed to a static basic magnetic field (e.g., the B0 field) that is as homogeneous as possible. The static basic magnetic field is produced by a basic field magnet in the magnetic resonance measurement device. Overlaid on this basic magnetic field during the recording of the magnetic resonance images are rapidly switched gradient fields that are produced by gradient coils. Apart from these, HF pulses with a defined field strength are radiated into the investigation space, in which the object under investigation is located, using a high-frequency antenna. The magnetic flux density of these HF pulses may be designated B1. The pulsed high-frequency field may thus also be designated the B1 field for short. Using these HF pulses, the nuclear spins of the atoms in the object under investigation are excited such that the nuclear spins are turned from an equilibrium orientation that lies parallel to the basic magnetic field B0, by an “excitation flip angle” (in what follows, also referred to as the “flip angle” for short). The nuclear spin precesses about the direction of the basic magnetic field B0. The magnetic resonance signals thereby produced are picked up by high-frequency receiving antennas. The receiving antennas may either be the same antennas as those, with which the high-frequency pulses are also emitted, or may be separate receiving antennas. The magnetic resonance images are generated on the basis of the magnetic resonance signals received.
The gradient coils have high currents (e.g., in the region of a few hundred Amperes) flowing through. The gradient coils are located in the strong basic magnetic field, in the immediate neighborhood of the basic field magnets. The interaction between the magnetic field from the gradient coils and the basic magnetic field is thus strong, and the mechanical effects on the gradient coils are known. Displacements of the gradient coils lead to known loud noises during magnetic resonance tomography. While these noises are perceived as disturbing, the noises are, however, neither damaging for the tomography system nor for the patient. The matter looks different when rapid gradient pulse sequences trigger mechanical resonance in the gradient coils or the gradient coil system, as applicable. Here, the noise is so loud that the noise is no longer acceptable for the patient. In addition, the mechanical vibrations have an effect on the gradient coil supply cables and on the gradient coil connections. As a result, for example, the screw connections on the gradient coils may become high resistance and, during longer periods of operation, heat up. Frictional heat may also result in warming. In an extreme case, the warming may become so intense that the warming leads to damage to the device.
Strong mechanical vibrations, such as arise in the case of a resonance, may also lead to a breakage in the coils or in the supply cables or in the devices for fixing the gradient coils.
The problem of mechanical resonance in the gradient coils arises, for example, in the case of very rapid magnetic resonance tomography imaging technology (e.g., with echo planar imaging (EPI)). Echo planar imaging is a one-shot method. In other words, for the measurement of one complete layer, an EPI sequence uses only one single excitation pulse. The switching of the gradient coils is bipolar in order to generate a plurality of gradient echoes with alternating sign. Apart from the mechanical effects of a resonant vibration of the gradient coil system, already described, in the case of echo planar imaging resonance, the resonance may also lead to artifacts in the image in the form of EPI ghosting (e.g., ghost image formation).
Mechanical resonance in the gradient coils also has an effect on the basic magnets. In a magnetic resonance tomography system, the basic magnet may be in the form of super-conducting coils. The super-conductivity is maintained by helium cooling. Resonance may cause warming, resulting in an increased rate of helium evaporation. In the extreme situation, the increased rate of evaporation may lead to magnetic quenching if the helium supply falls below the critical quantity. This provides that the superconductivity collapses, and the basic magnetic field is no longer maintained.
Resonance in a mechanical system, such as that represented by the gradient coils of a magnetic resonance tomography system, may arise at one or more resonant frequencies. The resonant frequency (frequencies) may be determined by measurement techniques or by a simulation.
The mechanical displacement of a coil, through which an electric current is flowing, is proportional to the current. Hence, a mechanical resonance with a resonant frequency f is evoked by an alternating current with a frequency f, provided that the magnitude of the current is sufficient to excite the mechanical vibrations by a magnetic field.
A first approach to avoid mechanical resonances in a coil provides that certain frequency bands that lie at the frequency of mechanical resonance or around the frequency of mechanical resonance are excluded or forbidden for the current flowing through. In magnetic resonance tomography systems, forbidden frequency bands of this type may be filed in the gradient pulse software. The software then prevents the possibility of executing gradient pulse sequences that contain frequencies within the forbidden frequency bands.
A disadvantage of this approach is that the approach makes all sequences that contain frequencies within the forbidden frequency bands unexecutable. The amplitude of the resonance excitation is not taken into account. This provides that some sequences, for which the resonance excitation amplitude lies below the critical value, become unexecutable (e.g., would not trigger any mechanical resonance).
In order to be able to generate optimal images using magnetic resonance tomography, clinicians themselves develop pulse sequences that are adapted to a specific investigation situation or a specific patient. The filing of forbidden frequency bands restricts the imaging options unnecessarily. If the user circumvents the frequency band bans, for example, because the amplitude has incorrectly been estimated to be adequately low, this may still lead to resonance excitation and, as a consequence, to damage to the gradient coil system.